The eye of Aplysia contains a beautifully precise circadian oscillator. This pacemaker is entrainable by light cycles, freeruns in constant conditions and plays a role in the control of a rhythmic behavior - the locomotor rhythm. Although the isolated eye can be entrained by light cycles and thus contains photoreceptors sufficient for phase control, extraocular pathways also appear to participate in the entrainment process. One goal of the proposed research is to determine whether photoreceptors comprise part of the extraocular pathways influencing ocular entrainment. Methods of procedure will include local illumination of putative photoreceptors in ganglia maintained in vitro, occlusion and microlesions to suspected receptors and intracellular and extracellular electrophysiological recording. The identification and subsequent localization of extraocular photoreceptors for ocular entrainment may lend the traditional virtues of the Aplysia nervous system (e.g., large, oftentimes identifiable neurons) to the study of the entrainment process, thus allowing continuous monitoring of photoreceptors during entrainment by light cycles. The second goal of the proposed research is to localize and characterize the pathways by which the ocular pacemakers control locomotor behavior. Three related approaches will be undertaken. In vitro recording from visual interneurons in the cerebral ganglion will aim at evaluating transformations in ocular signals during further processing, while restricted lesions to interganglionic connectives should localize critical pathways carrying ocular phase information. Finally, en passant recording of optic nerve activity in freely-moving Aplysia should reveal the relevant parameters of optic nerve activity in the initiation and control of locomotor behavior. Information gained from these three studies should lay the groundwork for a cellular based explanation of circadian control of locomotor behavior. Understanding of the neural mechanisms subserving the entrainment process and evaluating how circadian pacemakers control activity in the sensory-motor system provide two fundamental questions in studying the neurophysiological basis of circadian rhythms.